In the magical island of Nowhere, the lost boy was trying to find the entrance to the celestial garden he saw from a hill. A high wall surrounded the garden. Walking around, the boy found a door, very tall, with 12 stone locks placed vertically, from top to bottom, and so thin that they looked like stone strings. In fact, when touched, the stone locks vibrate, producing a beautiful note just like a crystal string singing. He also found that the tones in the stone were a semitone apart, just like a regular musical keyboard.
The boy liked to play music, so he played some melodies with that natural instrument. After some time, he remembered that he wanted to enter the garden, so looked for a sign. There was one that said, "The melody is the key to the beauty in the heart". At that moment, he remembered that the phrase was an old melody his grandma used to whistle to him (he was also a great melody whistler), and then figured out that that melody was the key to open the door. So he started touching the stones, playing the melody, all but the last note, because it was the highest stone in the door and he was too little to reach it. If only he could find a way to make the stone vibrate without having to touch it...
How could that be done?

Hint

Answer

Since he was a very good whistler, he just had to whistle a note one octave below the one that he wanted to vibrate. That stone will vibrate by the harmonics effect, in which the waves made from one frequency make another frequency to sympathetically "respond", thus making the other source vibrate a little. The second harmonic is the one that is double the base frequency, that is, one octave above.
Needless to say, the boy was a good musician and knew about the harmonics physics.Hide